利用蒙特卡罗模拟比较医用电子直线加速器均整模式和非均整模式下光子束的半影分布特性

Monte Carlo simulation and comparison for the penumbra characteristics of flattening filter and flattening filter free photon beams from medical electron linear accelerator

目的:通过蒙特卡罗模拟对比分析不同能量条件下光子均整(FF)射束和非均整(FFF)射束的半影分布特性。方法:利用BEAMnrc程序构建医科达Versa HD加速器FF模式和FFF模式下的机头模型,同时使用DOSXYZnrc程序建立标准水箱模型,模拟计算不同模式、不同打靶电子能量下的光子束在标准水模中的百分深度剂量(PDD)和射野离轴比(OAR),并与测量数据进行对比,确定不同模式下光子束的打靶电子能量。对FF射束采用射束中心轴剂量20%~80%之间的宽度计算半影,对FFF射束采用Ponisch方法计算半影,比较不同射束的半影宽度。结果:相同打靶电子能量条件下,6 MV FFF射束的半影相较于6 MV FF射束更锐利,但随着射野尺寸的增大,FFF射束的半影优势逐渐消失。当打靶电子能量提高到7.0 MeV时,6 MV FFF射束的半影与6 MV FF射束的半影趋于一致。对于10 MV FF和10 MV FFF射束,小野时FFF射束半影有明显优势。结论:打靶电子能量是决定不同射束半影特性的主要因素。

Objective To analyze the penumbral characteristics of flattening filter(FF)and flattening filter free(FFF)photon beams under different energy conditions using Monte Carlo simulation.Methods BEAMnrc program was used to model Elekta Versa HD accelerator head in FF mode and FFF mode,and DOSXYZnrc program to build the standard water tank model.The percentage depth dose(PDD)and off-axis ratio(OAR)of the photon beam in the standard water model under different modes and different incident electron energies were simulated and compared with the measurement data to determine the incident electron energy of the photon beam in different modes.For FF beam,the width between 20%and 80%of the beam center axis dose was used to calculate the penumbral;and for FFF beam,the penumbral was calculated with Ponisch method.The penumbral widths of different beams were compared.Results With the same incident electron energy,the penumbra of 6 MV FFF beam was sharper than that of 6 MV FF beam,but the penumbra advantage of FFF beam gradually disappeared with the increase of the field size.When the incident electron energy was increased to 7.0 MeV,the penumbra of 6 MV FFF beam tended to coincide with that of 6 MV FF beam.For 10 MV FF and 10 MV FFF beams,FFF beam penumbra had obvious advantages for small filed.Conclusion Incident electron energy is the main factor determining the characteristics of different beam penumbras.